Alt-BEAM Archive
Message #03256
To: Chiu-Yuan Fang chiumanfu@home.com,
From: Sean Rigter rigter@cafe.net
Date: Thu, 13 May 1999 22:41:25 -0700
Subject: [alt-beam] Servo Bots / Inspiration
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Hello all,
I had no idea of the volume of stuff I have written since I joined the
list 6 months ago. I definitely take up the bandwidth but it seems to be
appreciated. Chiu, thank you for organizing Rigter's Nupages. It would
have bogged me down a lot to do this myself and just while i'm on a
"creative roll".
So when the subject on the list touched on my current research project I
thought I better go on record.
So as a little taste of things to come here are three schematics for a
new class of Beam Tech : Beam Servo Bots. I have been working on the
idea of using hobby servos, without ripping their guts out, for a long
time but only recently had the time to try it out. These three servo
controller circuits use hobby servos the way they were designed to be
used but controlled in true Beam fashion with Microcore, Bicore and many
other Beam like circuits. No proprietary circuits or microcontrollers
just plain old Beam tech. This could blow the pants of low end RC market
as well: the implications are truly amazing. Imagine 6 "voltage to
servo" controllers using a single 74HC14. No more springs or limit
switches! Proportional controlled steering!
Hobby servos use pulses to adjust the angular position of the output
shaft. According to the standard these servos are designed to rotate
approximately 180 degrees. They require a pulse train of positive
pulses with a width of 300us to 2.5ms for maximum clockwise and counter
clockwise rotation. The motor hits the mechanical limits beyond these
values and this can damage the gear box as well as damage the
electronics. (so warning to all: there is no guarantee that these
circuits will work for you and the experimenter uses these circuits at
his own risk).
Servo Heads - proportional control
Hobby servos operate in two modes, digital mode and proportional
(analogue) mode. The Beam servo controller servohead1 schematic shows
full-blown linear voltage to pulse width converter using a 74HC14
Schmitt trigger oscillator which take the output of a pair LDRs, reverse
(green) LEDs, or PDs and proportionally positions the servo output shaft
controlled by the voltage at the midpoint of the "optical half bridge"
(OHB). This OHB approach is nice in that the output voltage is
proportional to the difference in light level falling on the sensor pair
and is virtually independent of the ambient light level. Servohead2 uses
the same idea but with a using a 74HC04 or 74HC240 cmos oscillator.
This oscillator is a close cousin to the Bicore but uses less
components. Both of these circuit use the same principle to control of
the pulse width with a control voltage by converting the control voltage
to a current with the transistor current source! When a transistor has a
resistor in the emitter, the current sourced from the collector is
proportional to the base voltage. Only the time constant of the positive
pulse is controlled with the control voltage. The time between pulses is
roughly 2.5ms but can vary a lot. This is not important in this
application as the servo is not sensitive to small variations in the
pulse frequency. For these oscillators the current source linearizes the
relationship between control voltage and pulse width and this is
important to be able to predict the final position of the servo rotor
for a given input voltage.
The action of the Servo Heads is a little different from the
conventional Beam heads in that they are more like the head of living
creatures turning 180 degrees and not like the spinning top (bring in
the exorcist) Beam heads rotating 360 degrees or more. This is nice
because the head is less likely to strangle itself on it's umbilical
cord (not a problem for autonomous or solar Beam heads also) To
stabilize the rotation avoid the shakes and generally reduce power I add
a cap to the midpoint of the OHB usually 1000pf for LEDs 0.1 for PDs and
up to 500uf for LDRs. Also watch the current consumption of the PDs and
LDRs, if very sensitive they can draw a lot of current and should be
protected with a 1K resistor in series with the OHB +V connection. Green
LEDs never have this problem and since the input current for the
Proportional Beam Servo Controller is extremely low it is perfectly
compatible with the small (ua) reverse LED currents.
Another application for this Proportional Beam Servo Controller circuit
would be as the proverbial hothouse controller using two thermistors in
the half bridge to control a vent based on the difference in
temperature. Or use a single thermistor and a resistor or a LM34 or
similar to control vents and valves for the VAC of your hothouse or home
or ??
Servo Core - Digital Servo Controller
Beam circuits require a complimentary power output stage like a BiCore
with a bridge or 2 microcore stages with 245 buffers for each motor.
Beam Servo Bots take advantage of the hobby servo build in electronics
power output circuit. In the Servo Core example a conventional ucore has
been adapted by greatly reducing the time constant of each stage to
generate 4 servo pulses which are send to 4 servos with the pulse width
of the pulses controlled via 4 control voltages. These control voltages
are generated by a second conventional 4Nv microcore as used in 2 motor
walkers. Notice that only one microcore stage is used for each motor
and that the digital output of each Nv supplies the fully left and fully
right signals.
As a result when the Nv output is high (75% of the time) the servo is
turned full clockwise and when the pulse goes low (25% of the time) the
servo slews to the counter clock position and return to it clockwise
resting position after the low pulse terminates.
The effect of this is a wave like (millipede) motion of the servos as
the process goes around the microcore loop. Of course this can be
extended to a larger number of Nvs and servos. I haven't used this on a
platform but it seems like a useful type motion. The 4 servo core is
just a starting point which can be connected to master slave Bicore and
even a microcontroller outputs to provide a convenient digital to servo
interface
Well I will stop here although there is much more but I am sure there
will be many other contributions to this fascinating application of Beam
tech to Servo Control.
Thanks to all the great ideas on this list : they truly are an
inspiration for me (and I am sure for Mark Tilden as well).
A special thanks to Steven Bolt for the idea of the current source to
control pulse width , the optical bridge and his many other unique
contributions. And of course a many thanks to Mark Tilden for starting
so much of this, staying at the center and somehow keeping the magic
going.
enjoy
Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
Chiu-Yuan Fang wrote:
>
> http://www.geocities.com/SouthBeach/6897/rigter/wr_1.html#jan18
> Wilf posted this message about driving steppers.
Dennison
> I have/had a bicore controlled RC car. It's kinda neat. I used a bicore > to control the forward/backward motiong, and another suspended bicore to
> controll the rack and pinion. So it steers twards the light.
James Taylor wrote:
> attach the steering servo to a light seeking bicore,
> you could easily make a phototropic car.
> I was thinking about this the other day. if you
> attach the steering servo to a light seeking bicore,
> you could easily make a phototropic car.
Josh
> I just got some great inspiration to turn my Radio Controled Porsche
> into a PhotoTroph.
> No I haven't learned all that but I will eventually turn it into
> a Phototroph i am studying its circuts and figuring out how to add
> photodiodes and such. Also Nv's for aestetics I was going to put the
> Photo Diode on the Tail and Head Light Holes :) Mail me with any
> suggestions, Idea,or tips. Thanx !
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3257 Thu, 13 May 1999 23:36:20 -0600 [alt-beam] Re: Inspiration LiquidFord@aol.com, aubois@trail.com Dave Hrynkiw At 09:14 PM 5/13/1999 , LiquidFord@aol.com wrote:
>Uhh... Hmm... Sorry I hope it will seek the Visual and Near Infrared
>spectrums of Light. This means turns towards and going forwards towards any
>the stated above spectrum of light.
Don't let minor semantics get in the way of what you're trying to describe.
We perfectly understand what you meant. Good luck with the project.
...I think I'll just get back to working on my Aquavore....
-Dave
---------------------------------------------------------------
"Um, no - that's H,R,Y,N,K,I,W. No, not K,I,U,U, K,I,_W_. Yes,
that's right. Yes, I know it looks like "HOCKYRINK." Yup, only
2 vowels. Pronounciation? _SMITH_".
http://www.solarbotics.com
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